Combustors are used to ignite and burn fuel and air mixtures in gas turbine engines. Known combustors include inner and outer liners that define an annular combustion chamber in which the fuel and air mixtures are combusted. The inner and outer liners are spaced radially inwardly from a combustor casing such that inner and outer passageways are defined between the respective inner and outer liners and the combustor casing. Fuel igniters extend through the combustor casing and the outer passageway, and are coupled to the outer liner by igniter tubes attached to the combustor liner. More specifically, the fuel igniter tubes secure and maintain the igniters in alignment relative to the combustion chamber as well as provide a sealing interface for the igniter between the outer passageway and the combustion chamber.
During operation, high pressure airflow is discharged from a compressor into the combustor. A portion of the airflow enters the combustion chamber, where it is mixed with fuel and ignited by the igniters. Another portion of the airflow entering the combustor is channeled through the combustor outer passageway for attempting to cool the outer liner, the igniters, and diluting a main combustion zone within the combustion chamber. However, over time, continued operation may induce potentially damaging thermal stresses into the combustor that exceed an ultimate strength of materials used in fabricating the components of the combustor. For example, thermally induced transient and steady state stresses may cause low cycle fatigue (LCF) failure of the igniter.
Cooling the igniter, particularly the tip portion of the igniter closest to the combustion process, presents challenges. Some conventional igniters include a plurality of longitudinal slots extending down the length of the igniter to channel cooling air to the vicinity of the tip portion of the igniter. However, this arrangement is not very efficient because it requires a relatively large amount of cooling air to sufficiently cool the tip portion of the igniter. The large amount of air required to effectively cool the tip portion of the igniter in this manner may adversely impact the combustion conditions within the combustion chamber. Particularly, a large amount of cooling air may have a perturbative effect on the ignition process, gaseous emissions, and the temperature distribution of hot gases entering the turbine. In some arrangements, the quantity and manner in which cooling air is admitted into the combustor may result in a barrier formed around the igniter that prevents fuel from reaching the tip portion of the igniter. This can additionally reduce the effectiveness of the igniter for igniting the fuel and air mixture. Moreover, excess cooling air can disrupt the liner cooling film and result in local hot spots immediately downstream of the igniter in the combustor liner.
Accordingly, it is desirable to provide combustors with igniters that are efficiently cooled without adversely interfering with the combustion of the air and fuel mixtures in the combustion chamber. In addition, it is desirable to provide igniter tubes for improved cooling of igniters in combustors. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.